Fibre Reinforced Facade Encyclopedia: Manufacturing
A flex anchor is the slender steel rod that connects a thin glass fibre reinforced concrete (GRC, GFRC) skin to the steel stud frame behind it. It carries wind load out of the plane of the panel and it carries nothing in the plane of the panel. Its free length is deliberately made long and thin enough to bend, so that when the concrete skin expands, contracts or moves differently from the steel frame, the anchor deflects instead of resisting. The flexibility is not a tolerance allowance. It is the entire reason the anchor exists.
A flex anchor is a rod that is supposed to bend. If it is too short, too thick, or grouted solid at the frame, it stops being a flex anchor and becomes a crack initiator.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Function | Wind load out of plane only | No in-plane load, ever |
| Rod | 6 to 10 mm diameter, austenitic stainless steel | |
| Free length | 40 to 100 mm typical | The unrestrained length that provides the flexibility |
| Bonding pad | About 100 x 100 mm, 15 to 20 mm thick | Sprayed over the anchor leg, wet on wet into the green skin |
| Spacing | 400 to 600 mm grid typical | |
| GRC moisture movement | 0.5 to 1.5 mm per metre, dry to saturated | Steel frame does not move with moisture at all |
Why a thin skin must be allowed to move
A stud frame panel is two materials with different behaviour bolted together. The glass fibre reinforced concrete skin is 13 to 15 mm thick, sits on the outside of the building, and sees the full temperature range plus the full wetting and drying cycle. The steel stud frame sits in the cavity, shaded and dry, at a temperature that lags the skin and a moisture content that never changes.
The numbers are easy to underestimate. GRC moisture movement between dry and saturated is of the order of 0.5 to 1.5 mm per metre. Thermal movement at 10 to 20 x 10-6 per degree, over a surface temperature swing that on a dark panel can exceed 60 degrees, adds a further 0.6 to 1.2 mm per metre. Over a 3 metre panel that is several millimetres of movement in the plane of the skin, relative to a frame that is not moving with it.
Restrain that movement and the skin cracks. Its limit of proportionality, the stress at first crack, is typically 7 to 11 MPa for sprayed GRC, and it takes very little restraint to reach that. A restrained panel cracks at the anchor positions, at reentrant corners and around openings, permanently and visibly. This is the most common cause of cracking in stud frame GRC panels, and it is a detailing failure rather than a material failure.
How the anchor achieves the flexibility
The anchor is a steel rod bent to an L or a Z. One leg lies against the back of the skin and is encapsulated in a bonding pad, a small patch of the same GRC material sprayed over it. The other leg is welded or bolted to the stud frame. Between the face of the pad and the face of the frame there is a free length of rod, unrestrained and unsupported, typically 40 to 100 mm.
That free length is a cantilever. Push the skin sideways relative to the frame and the rod bends in double curvature, and the force required is small because bending stiffness falls with the cube of the free length. A 6 mm rod with a 60 mm free length accommodates several millimetres of in-plane movement at a force the skin absorbs without distress. A 12 mm rod with a 20 mm free length does not; it transfers the restraint into the pad and cracks the skin around it.
Out of plane the same rod is stiff enough, because the load path is axial rather than bending: wind pressure puts it in compression and suction puts it in tension. The same component is deliberately soft in one direction and stiff in the other.
Detailing and layout rules
- Arrange the anchors so the skin can expand radially about the gravity anchor position rather than being locked between fixed points
- Keep the anchor grid to the spacing the panel design requires, typically 400 to 600 mm, with anchors set back from panel edges and from ribs
- Spray the bonding pad wet on wet into a green skin, never onto a cured surface, so it becomes part of the skin rather than something stuck to it
- Keep the pad to its designed size and thickness. An over-thick pad shrinks differently from the skin around it and shows as a mark on the face
- Leave the free length genuinely free: no grout, foam, packing, insulation or sealant between the pad and the frame
- Keep the frame clear of the back of the skin except at the anchors, so it cannot restrain the skin by contact, and never weld to an anchor after its pad is cast
Specification and verification
Anchor material is selected for the environment, not for convenience. The anchors sit in a damp cavity for the design life of the facade and can be neither inspected nor replaced. Austenitic stainless steel is the normal choice, with the grade set by the atmospheric corrosivity category of the site. A corroding anchor expands and spalls the pad off the back of the skin, which is a panel failure, not a defect. Pull-off testing of sample pads verifies the design at the prototyping and first-off panel stage. The panel fixing systems page covers how the frame is then connected to the building.
Standards and references
- GRCA Practical Design Guide for the design of GRC stud frame panels and anchor systems
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products
- BS EN 1169 factory production control and BS EN 1170-5 for the bending properties the pad design relies on
- PCI MNL-128 Recommended Practice for Glass Fiber Reinforced Concrete Panels (United States anchor practice)
- AS/NZS 1170.2 wind actions, for the out of plane design load on the anchor
Related terms
Questions specifiers ask
What does a flex anchor do?
It ties a thin GFRC skin to its steel stud frame, transferring wind pressure and suction out of plane while bending freely in plane so that differential movement between the skin and the frame is not restrained.
Why must a flex anchor be flexible?
Because the skin and the frame move differently. GRC moves 0.5 to 1.5 mm per metre with moisture and a further 0.6 to 1.2 mm per metre thermally, while the frame in the cavity barely moves. Restraining that difference cracks a skin whose first crack stress is only 7 to 11 MPa.
What is the difference between a flex anchor and a gravity anchor?
Gravity anchors carry the self weight of the skin in plane, and there is one effective pair per panel. Flex anchors carry wind load out of plane only and no in-plane load at all.
What are flex anchors made from?
Austenitic stainless steel rod, typically 6 to 10 mm in diameter, with the grade selected for the atmospheric corrosivity category of the site. They cannot be reached or replaced once the panel is built.
Specifying a stud frame panel? ShapeShell-RC panels are engineered with their anchor layouts, pad details and frame connections resolved before the mould is cut. See ShapeShell-RC specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.